The lithography machine takes an important role in the manufacturing and production of semiconductor devices such as VLSIs, sensors, surface wave components, magnetic bubble devices, microwave devices, and CCDs, etc. In the photolithographic process, the patterns on the mask are transferred to the photoresist coated on the surface of the wafer by the exposure process, and then transferred to the wafer by developing and etching. The photolithographic process determines the critical dimension of the VLSI and is a critical process in the manufacturing of the VLSI.
The focus of the lithography machine is an important parameter of the product performance, which needs to be monitored regularly. The PSFM (Phase Shift Focus Monitor) reticle changes the vertical distance between the lights into a horizontal distance. Therefore, the focus difference of the lithography machine can be calculated by measuring the horizontal distance between the outer box and the inner box of the “box-in-box” overlay marks exposed on the wafer using an overlay measuring machine. As shown in FIG. 1, which is a schematic diagram of the conventional PSFM reticle, the PSFM reticle comprises a glass layer 1 having multiple openings 2. In the three dimensional space, the lights vertically incident on the reticle (in the direction of the z-axis perpendicular to the surface of the glass) passes through the reticle and converts into two beams of light, one beam of light passes through the air in the opening 2 of the glass and the other beam of light passes through the whole glass 1. Since the wavelength of the light in the air is different from that of the light in the glass, the two beams of lights may interfere with each other, and the wavefront of the interfered lights changes from an original horizontal direction (the x-axis direction or the y-axis direction) into an inclined direction (between the x-axis direction and the y-axis direction). Since the direction of the emergent lights is perpendicular to the wavefront, the normal incidence lights will be changed to oblique emergent lights after passing through the openings 2. Therefore, the shifts of the emergent lights in horizontal vector (x vector or y vector) are different in the horizontal planes of different heights.
After the exposure process for the reticle, the overlay marks corresponding to the openings of the reticle will be formed on the wafer, the focus difference of the lithography machine may cause the overlay difference. FIG. 2 is a schematic diagram illustrating the conventional phase shift focus monitor reticle and the conventional method of monitoring the focus difference using the phase shift focus monitor reticle. As shown in FIG. 2, the conventional reticle comprises a glass layer 1 having multiple openings 2, a chrome outer box 3 and a chrome inner box 4 positioned below the glass layer 1. The normal incidence lights passing through the glass layer 1 changes into oblique emergent lights due to the interference, the chrome outer box 3 and the chrome inner box 4 below the glass layer 1 will limit the emergent lights in a certain range (as shown by the arrows below the chrome outer box 3 and the chrome inner box 4 in FIG. 2), wherein the chrome outer box 3 and the chrome inner box 4 are opaque. As shown in the right part of FIG. 2, the chrome outer box 3 and the chrome inner box 4 are not perfectly symmetrical, the shift between the middle of the chrome outer box 3 and the middle of the chrome inner box 4 in the x-axis direction is Δx. In the horizontal plane of the Z0-axis, which has the BF (best focus) of the lithography machine, the shift (in the x direction or in the y direction) between the middle of the exposed patterns of the chrome outer box 3 and the chrome inner box 4 is measured to be Δ0 by the overlay measuring machine. If the focus of the lithography machine shifts from the plane of the Z0-axis to the plane of the Z1-axis, the measured shift between the middle of the exposed pattern of the chrome outer box 3 and the chrome inner box 4 in the plane of the Z1-axis will be changed into Δ1 since the emergent light is oblique. Similarly, the shift between the middle of the exposed patterns of the chrome outer box 3 and the chrome inner box 4 in the plane of the Z2-axis and that in the plane of the Z3-axis will be measured to be Δ2 and Δ3, respectively. Therefore, when the reticle is exposed on the wafer with different focus, the shifts between the middle of the exposed patterns of the chrome outer box 3 and the chrome inner box 4 measured by the overlay measuring machine can be modeled, and further the linear relationship between the shift of the lights along the z-axis and the shift of the lights along the x-axis or the y-axis can be calculated. In the regular monitoring for the focus of the lithography machine, the shift of the lights along the z-axis can be calculated according to the linear relationship and the shift between the middle of the exposed patterns of the chrome outer box 3 and the chrome inner box 4 measured at that time. Since the focus difference has a corresponding relationship with the shift of the lights along the z-axis, the difference between the present focus of the lithography machine and the best focus thereof can also be obtained accordingly.
Due to the high accuracy and repeatability of the overlay measuring machine, the precision can be improved and the measuring time can be reduced by the monitoring method mentioned above. However, since the normal lights can be changed into oblique lights only in a limited range close to the openings (within the chrome outer box 3) in the reticle structure as shown in FIG. 2, the exposed patterns on the wafer to be measured by the overlay measuring machine are narrow line marks, and the photoresist coated on the wafer has to be thin enough to prevent the collapse of the photoresist in the narrow line marks.